1. Field of the Invention
This invention relates to a stereophonic pickup cartridge of the moving coil type wherein coils are mounted on the rear part of a cantilever having a stylus attached to the front end thereof and wherein the coils are arranged within a magnetic field in a magnetic air gap.
In a pickup cartridge of this type, electric signals are generated in coils which are installed on an end of the cantilever opposite the stylus. The coils move with the stylus vibration within air gaps of a magnetic circuit which is formed by a magnet and yokes. The conversion of a stylus vibration into an electrical signal is direct while the electrical impedance of the cartridge is low scarcely varying in a region from low to high audio frequencies. Accordingly, the vibration of the stylus is proportionally changed to an electric signal, and a uniform sound reproduction of very high fidelity is possible. The pickup cartridge is therefore typically used with high-quality sound reproduction equipment.
2. Description of the Prior Art
A moving coil type stereophonic pickup cartridge which has heretofore been used extensively is shown in FIG. 1A of the accompanying drawings. A stylus 1 is mounted on the front end of a cantilever 2, while a square magnetic plate 3 is mounted orthogonally on the rear supporting end part of the cantilever 2. Two sets of coils 4, 4' and 5, 5' which intersect orthogonally to each other are wound on the magnetic plate 3.
Numeral 6 designates a magnet. The lower part of an angular front yoke 7 is attached to one end of the magnet 6. At the other end of the magnet 6, a rear yoke 8 is attached which is shorter than the front yoke 7. A cylindrical holder yoke 9 has its base part fixed in a manner to extend from the upper end of the rear yoke 8 towards the magnetic plate 3. A gap is formed between the holder yoke 9 and the magnetic plate 3. A damper 10 which is made of soft rubber, or the like, is arranged between the magnetic plate 3 and the holder yoke 9 so as to damp the vibration of the stylus 1.
In the cartridge thus constructed, the magnetic flux of the magnetic 6 can be said to flow through the front yoke 7 and the rear yoke 8 at both ends of the magnet 6 and the holder yoke 9. An electric current is induced in the coils 4, 4' and 5, 5' by movement of the coils in the magnetic field existing between front yoke 7 and holder yoke 9. Ideally, when one set of coils 4 and 4' interacts with the magnetic flux, the other set of coils 5 and 5' does not so that the respective signals of the right and left channels are detected independently. A further description of cartridges of this type can be found in British Pat. No. 1,000,035.
However, it has been found that this cartridge exhibits a low output level, thus necessitating a special amplifier or a step-up transformer. In general, the cartridge exhibits low trackability or compliance. In the fabrication of such a cartridge, many difficulties are involved in the winding of the coils, the terminal processing of coil leads, etc. These characteristics have hindered the commercial acceptance of such a cartridge.
With a pickup cartridge of the construction illustrated in FIG. 1A a higher output can be obtained by narrowing the gap between the holder yoke 9 and the front yoke 7 thus intensifying the magnetic field. This magnetic field enhancement is restricted, however, because the damper 10 exists within the gap.
Further, since the two independent signals of the right and left channels are obtained by arranging the two sets of coils intersecting orthogonally to each other within a single magnetic field in an identical direction, the independency of the two signals is determined by only the winding directions of the coils. This leads to crosstalk problems which are difficult to correct.
With respect to frequency characteristics, a short cantilever is generally thought to be advantageous because the vibrating system can be constructed of a lighter mass and has a lower moment of inertia. The shortening of the cantilever structure shown in FIG. 1 is restricted by the hindrance of the tip part of the front yoke 7.
Another example of the prior art is to be found in U.S. Pat. No. 3,679,843 wherein the square magnetic plate 3 illustrated in FIG. 1A is replaced by a round magnetic plate 3' illustrated in FIG. 1B having a pair of extensions 3" and 3"'. A single coil 4 and 5 is wound on each of the extensions 3" and 3"' respectively. The stylus subassembly thus formed is then positioned in a magnetic circuit nearly identical to that previously described. The axis of the two coils 4 and 5 remains orthogonal to each other which the coils are positioned in a single common magnetic field similar to that previously discussed. The use of a single common magnetic plate 3 continues to introduce unwanted crosstalk which is difficult to further reduce. Another example of prior art is U.S. Pat. No. 3,299,219.